The present invention relates to separation of maleic anhydride or phthalic anhydride from a gaseous mixture containing the anhydride, particularly by the use of an organic absorbent.
Phthalic anhydride and maleic anhydride are both important industrial chemicals which are produced by vaporphase oxidation of a hydrocarbon feedstock in an oxidation reactor, followed by recovery and then purification of the anhydride. The most common feedstocks for phthalic anhydride plants are naphthalene and orthoxylene; for maleic anhydride plants, the most common feedstock is benzene, although other hydrocarbon feeds have been disclosed, including butene and butane.
Recovery of the anhydride from the gaseous effluent stream from the oxidation reactor can be done by scrubbing the effluent with water, which results in conversion of the anhydride to an acid. The acid then needs to be dehydrated to produce the anhydride product.
In the case of phthalic anhydride (b.p. = 285.degree. C), condensation of the anhydride present in the reactor gaseous effluent can be done to separate the phthalic anhydride. Frequently maleic anhydride (b.p. = 202.degree. C) is recovered by the condensation of a part of the maleic anhydride and maleic acid in the gaseous effluent, the remainder being recovered by scrubbing with water, resulting in an aqueous maleic acid solution. Then both the partial condensate and the aqueous acid solution must be dehydrated to obtain product maleic anhydride.
Recovery of maleic anhydride or phthalic anhydride from the oxidation reactor effluent using an organic absorbent as opposed to an aqueous absorbent has also been disclosed. For example, U.S. Pat. No. 2,574,644 discloses the use of dibutylphthalate for the recovery of maleic anhydride or phthalic anhydride from an oxidation reactor effluent stream. According to the process disclosed in U.S. Pat. No. 2,574,644, the oxidation reactor effluent is cooled to first condense a portion of the anhydride vapor. The remaining gaseous stream is contacted with the dibutylphthalate absorbent to remove the remaining uncondensed anhydride by absorption into the absorbent. The resulting rich absorbent is stripped to obtain a product anhydride stream.
British Pat. No. 727,828 discloses the use of dibutylphthalate for simultaneous absorption of both maleic anhydride and phthalic anhydride at absorbent temperatures above 40.degree. C.
U.S. Pat. No. 3,040,059 discloses the use of molten wax as an absorbent for removing maleic anhydride or phthalic anhydride from an oxidation reactor effluent stream.
U.S. Pat. No. 2,893,924 discloses the use of diphenylpentachloride absorbent as well as tricresyl phosphate as an absorbent for removing maleic anhydride or phthalic anhydride by absorption.
Japanese Pat. No. 35-7460 discloses the use of dibutylmaleate as an organic absorbent for removing maleic anhydride from gas streams, and Japanese Pat. No. 32-8048 discloses the use of dimethylterephthalate for removing maleic anhydride from gas streams containing maleic anhydride.
U.S. Pat. No. 3,891,680 discloses the use of certain dialkylphthalates as absorbents for removal of maleic anhydride from gas streams containing maleic anhydride.
Also, U.S. Pat. No. 3,818,680 discloses alkyl or alkenyl succinic anhydrides, in general intramolecular carboxylic acid anhydrides, as absorbents for the removal of maleic anhydride from gas streams. The disclosures of U.S. Pat. Nos. 3,891,680 and 3,818,680, especially in that they relate to the use of organic absorbents for maleic anhydride removal, are incorporated herein by reference.
These prior art absorbents have drawbacks which have prevented their acceptance and use in commercial applications. The ester-type absorbents -- dialkylphthalates, maleates, etc. -- can undergo some transesterification with maleic anhydride and also can decompose to give some anhydride and alcohol; all of these by-products contaminate the maleic anhydride product. The polychlorinated biphenyls are hazardous to the environment and are essentially forbidden by law. Molten wax has limited solubility for maleic anhydride. As a consequence, commercial maleic anhydride plants use water recovery or a system or switch condensers, and no organic-media recovery units are presently in operation.
Typical prior art absorption schemes involve the use of what is frequently called a "figure-8 loop". The figure 8 is formed by the absorbent entering the top of the absorber, flowing downwardly therein, and then being transferred as rich absorbent to the absorber stripper, flowing downwardly in the stripper and then being transferred as lean absorbent to the upper part of the absorber to complete the figure-8 loop. In the absorber, the absorbent absorbs the solute and in the stripper the solute is distilled out of the absorbent. Distillation is typically accomplished by generating upflow vapors through heat input to the lower part of the stripper.
For example, the above-mentioned U.S. Pat. No. 2,574,644 shows an anhydride absorption scheme wherein basically a figure-8 loop is used.
A common problem in separation of a constituent from a gas mixture by countercurrent absorption in an absorber column is loss of absorbent out of the top of the column with the lean or scrubbed gas stream flowing out the top of the column. The loss is caused by the entrainment and vaporization of absorbent into the upwardly flowing lean gas stream. Entrainment can be reduced by using a deentrainment device such as a demister pad and by reducing gas flow rates through the column. Vapor losses of absorbent can be reduced by using a lower temperature, using a higher pressure, and use of less volatile solvents, if such solvents are available.
U.S. Pat. No. 3,850,758 shows the purification of crude maleic anhydride using dimethylbenzophenone by heating the maleic anhydride in the dimethylbenzophenone at a temperature above 230.degree. C and at total reflux for at least 15 minutes, and then recovering maleic anhydride by distillation. The U.S. Pat. No. 3,850,758 also mentions the use of dimethylbenzophenone as a recovery solvent for maleic anhydride produced by gas phase oxidation of organic compounds and employing the same dimethylbenzophenone in the purification process.